fix(cast): wait for media session before issuing play()

[brycesub]

Problem

Casting intermittently crashes at startup with:

pychromecast.error.RequestFailed: Failed to execute play.

play_media(autoplay=True) only starts playback once the device has fetched the stream and established a media session. That handshake is asynchronous, but play_cast() slept a fixed 5 seconds and then unconditionally called media_controller.play(). When the session wasn't active yet, play() raised RequestFailed and the unhandled exception killed the whole cast.

This is a pre-existing race — the sleep(5) + play() block dates back to 2024-02. It rarely triggered because most paths establish a session within 5 seconds, but a slow cold start (portal grant + encoder init + buffering) reliably exceeds it.

Fix

Replace the blind sleep with media_controller.block_until_active(timeout=30), which returns as soon as the session is ready. Then only call play() when a session is actually active (and skip it entirely if autoplay is already playing), printing an actionable warning instead of crashing if no session forms in time.

The fix is generic and benefits every cast path.

Tests

Adds tests/test_cast_play.py covering the three handshake outcomes:

• no session → must not crash, must not issue play()
• active session → must play()
• already playing → must not re-issue play()

$ python -m pytest tests/test_cast_play.py -q
...                                          [100%]
3 passed

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